Field of the Invention and Related Arts Statement
The present invention relates to optical fiber-containing insulators which can endure environmental stresses, for example, due to temperature variations of the environments to be used and which can maintain air-tight properties for a long time. The present invention also relates to a process thereof.
In power substations, power-transmission lines and power-distribution lines, it is desired to provide a system for quickly detecting faults in electric power systems and for immediately repairing these faults. Usually, there has been known an apparatus for detecting unusual currents or unusual voltages which utilizes an optical sensor having characteristics such as Faraday effects and Pockels effects. Moreover, in order to insulate the transmission current or the transmission voltage between a fault locator and the optical sensor arranged to a conductor of the normally power supplying side, use is made of an insulator. Therefore, it is necessary to use the optical fiber-containing insulator which transmits only optical signals while maintaining the electric insulating properties. In the optical fiber-containing insulator used for this purpose, it is necessary to maintain a mechanical strength and an electric insulation property for a long time in addition to the transmission properties of the optical signals.
In the optical fiber-containing insulators, as to sealing materials for maintaining air-tight properties between the optical fiber and a porcelain, use is made of an organic material or an inorganic material such as glass, and both of them have special merits and demerits, respectively.
When use is made of an organic material such as silicone rubber as the sealing material, it is possible to seal airtightly a space between the optical fiber and the porcelain and to perform a sealing operation easily in low temperatures, and thus there is a merit such that a manufacturing of the optical fiber-containing insulator can be performed easily in a large scale.
Contrary to this, when use is made of an inorganic material such as glass as the sealing material, the inorganic material has excellent weatherproof and chemical corrosionproof properties in comparison with the organic material and thus if material characteristics such as thermal expansion coefficients of the inorganic material meet with those of the surrounding material, it is possible to achieve an excellent long reliability on the air-tight and insulation properties.
In the optical fiber-containing insulator in which the organic material is used as the sealing material, since thermal expansion coefficients between the organic material and the porcelain are largely different from each other in a general case, there is a drawback such that there occurs a deterioration of the organic material used as the sealing material due to a temperature variation generated in the environmental temperatures and a breakage of the optical fiber due to the deterioration of the organic material. Moreover, since the organic material is deteriorated due to a tracking generated therein for a long term, it is preferred that, as to the sealing material for maintaining the air-tight and insulation properties, use is made of the inorganic material such as glass having the substantially same thermal expansion coefficient as that of the porcelain.
Contrary to this, in the optical fiber-containing insulator using the glass mentioned above, since a large furnace must be used so as to melt the glass by heating the whole porcelain, there is a drawback such that an equipment cost increases and a cost of power consumption also increases.
Further, as to the optical fiber used in the optical fiber-containing insulator, it is necessary to use the optical fiber made of quartz glass in view of the heat resistive properties. The mechanical strength of the optical fiber is maintained by covering an organic material as a covering layer such as silicone resin, urethane resin and epoxy resin on an outer surface of threads just after spinning threads out of another material made of quartz glass. Since the mechanical strength of the optical fiber is extremely low if there is no covering layer, the covering layer must be arranged on the cuter surface of the optical fiber to handle it.
However, such an organic covering layer does not basically endure a heat treatment performed above 300.degree. C. Therefore, if the organic covering layer is once deteriorated by the heat treatment etc., the optical fiber becomes very brittle and thus it is broken under extremely low strength than that before the heat treatment. For example, when a diameter of the optical fiber is 125 .mu.m, the breaking load of the optical fiber is 6 kg before the heat treatment and is 0.5 kg after the heat treatment.
Accordingly, when the whole insulator is heated so as to melt the glass materials for the sealing, the strength of a part of the optical insulator projected from the porcelain becomes low and then the optical fiber is broken in the next step. Therefore, after sealing the optical fibers in the porcelain, the opposed end surfaces of the insulator containing the optical fibers are mirror-polished and then it is necessary to connect optically the other optical fibers to the bared optical fibers in the insulator by using ferrules. In this case, there occurs a drawback such that the manufacturing steps become complicated and thus the manufacturing cost increases.